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A micro rod machining method which can switch between electrical discharge machining (EDM) and electrochemical machining (ECM) by attaching/detaching a diode to/from a bipolar pulse generator in parallel to the working gap was newly developed using a wire electrode made of tungsten. The problem of the wire electrode wear was eliminated by the use of the wire electrochemical turning (WECT) method in which the tungsten wire electrode is continuously running. The ultra-short bipolar pulse current was generated by the electrostatic induction feeding method where a pulse voltage is coupled to the working gap through a feeding capacitance. The machining characteristics of three types of wire guide; disk-shaped WC guide, laminated wire guide and cylindrical acrylic guide, were studied. The experimental results showed that the cylindrical acrylic guide has the best machining characteristics without the influence of guide wear and with less stray current flowing through the working gap. Using the cylindrical acrylic guide, the influences of the feeding capacitance C1, and the total amplitude of the pulse voltage on the machining characteristics were studied. Finally, a stainless steel SUS 304 micro-rod with a high aspect ratio of 14 was fabricated efficiently by using the EDM and ECM modes for rough and finish machining in sequence with the same setup, pulse generator, and neutral electrolyte.

Electrolyte Jet Machining (EJM) has been performed on stainless steel surfaces with the aim of reducing bacterial retention through the generation of nanoscale surface morphology. Following initial EJM experiments aimed at investigating the influence of machining depth, machining speed and current density on the resulting surface roughness, three characteristic surfaces were produced with a current density of 10–18 A/cm2 and a machining speed of 0.8–8 mm/s to obtain an arithmetic mean height (Sa) of 0.5–0.74 μm and a density of peaks (Spd) of 0.25–1.26 μm−1. Relatively large differences between the three surfaces in terms of Spd allowed thorough investigation into the effects of surface feature size on bacterial retention to be performed. Reductions in the order of 90% compared to control samples were achieved for gram-positive Bacillus cereus and Staphylococcus aureus across the entire tested parameter range (Spd = 0.25–1.26 μm−1), while reductions in the order of 99% were achieved for gram-negative Escherichia coli and Pseudomonas aeruginosa for surfaces characterized by Spd > 1 μm−1. Not only do the results call attention to EJM as an innovative technology for producing antibacterial surfaces, they also highlight important differences in the behavior of gram-positive and gram-negative bacteria in relation to EJM-textured surfaces with nanoscale surface morphology.

This paper introduces a newly developed EDM pulse generator whose high open voltage for discharge is generated by induced electromotive force using a power supply of only 5 V. The pulse generator can generate short duration and high discharge current pulses. The rising time of discharge current pulses to 5 A can be made shorter than 50 ns with a pulse duration of 200 ns or shorter. The results proved that such discharge current pulse shapes increase material removal volume per discharge and that the higher boosting open voltage facilitates the machining of high resistivity materials.

The novel wire electrochemical turning (WECT) method has been proposed to machine tungsten with neutral electrolyte and bipolar pulse current in the authors’ previous research. The influence of tool wear caused by the bipolar pulse current, which is needed to remove the oxide layer generated on tungsten surface when using a neutral electrolyte, was eliminated by the WECT method because the wire tool electrode was kept being wound during the machining process. In this paper, the influences of the most important process parameters, such as the kind of electrolyte, wire material, wire diameter, rotation speed of the workpiece rod, and frequency of the pulse voltage, were studied to understand the fundamental aspects of the WECT method and improve the machining efficiency of tungsten micro-rods. The experimental results showed that the NaCl electrolyte and the NaNO3 electrolyte had higher machining efficiency and better machining accuracy, respectively. The machining accuracy was improved with the SUS304 stainless steel wire used as tool electrode than the brass wire tool electrode. It was also found that the machining efficiency and accuracy were improved with decreasing the wire diameter and increasing the rotation speed of workpiece because the influence of stray current flowing through the side and end surface of machined micro-rod was reduced. The machinable length of micro-rod peaked at an optimum feed speed of the workpiece, and the optimum speed increased with increasing the frequency of pulse voltage. As a result, a minimum rod diameter of 11 μm was fabricated with a high aspect ratio of 36. Compared with the previous method using a platinum film as tool electrode, the aspect ratio was increased significantly because the influence of tool wear was eliminated.

It is known that only several percent of the meltpool in the discharge crater is removed as debris in electrical discharge machining (EDM). Hence, machining rate would be improved and thickness of the damaged layer would be decreased if most of the molten material could be removed. In the previous researches, it was found that the removal volume of single pulse discharge, machining rate of continuous pulse discharges, and tool wear are significantly influenced by the rising and falling speeds of the discharge current pulse. In the previous researches however, the influences of the pulse shapes were investigated under a limited machining condition. Machining characteristics of these pulse shapes have not been compared under different machining conditions. This study shows that the most efficient pulse shape can be different if the discharge duration and discharge current are changed. The reason why the optimum pulse shape depends on the machining conditions is discussed.

In wire electrical discharge machining (WEDM), discharge craters and heat-affected zone impair the surface roughness and generate micro cracks and residual stress. Hence, this study aimed to finish the WEDMed surface by electrochemical machining (ECM) using the same setup, wire electrode and machining fluid used in WEDM. Ultrashort bipolar pulses were used to improve the finishing speed and surface roughness. To realize electrochemical finishing on a WEDM machine, first the influences of the electrolyte concentration on machining characteristics were investigated, and it was found that machining can be performed even if the electrolyte concentration of NaNO3 aqueous solution is diluted to 0.2 wt%. With lower electrolyte concentration, smaller surface roughness was obtained at smaller machining depth. Next, the material of the wire electrode was changed from platinum to brass which is commonly used for WEDM, and the wear rate of the brass wire was investigated. Although the wear rate was 20 times higher than that of platinum, wire breakage did not occur in most of the experiments conducted in the present work. Then, machining was performed with an equal current density for different thicknesses of workpiece, and influences of the workpiece thickness on the process characteristics were investigated.

The increasing demand for optimized component surfaces with enhanced tribological, optical, chemical, biomedical, and geometric complexity is a key driver in the development of electrolyte jet machining (EJM) technology. To date the applications of EJM method have been limited to planar surface processing. Realization of three-dimensional freeform surfaces with high precision by EJM is of significant importance to advance the process. In this study, the characteristics and principle of multi-axis EJM of arbitrary-shaped workpieces were investigated for the first time. The resultant machined features were experimentally demonstrated under principal factors of inter-electrode gap distance, jet orientation relative to workpiece surface, jet orientation relative to gravity, and workpiece configuration. Furthermore, a simulation model of EJM considering jet orientation and workpiece curvature was built to analyze the electrolyte jet flow and clarify the principle of the current density distribution in the non-symmetric electrolyte flow. It is demonstrated that jet orientations directly affect the profiles of current density distribution and consequently result in varied machining results. In EJM of freeform surfaces, a smaller radius of curvature brings about more intensive concentration of current density distribution and consequently a larger machining depth. The same principles are confirmed in EJM of corner shapes with different angles.

A novel method to switch the machining mode between micro-EDM and micro-ECM in the same electrolyte using the same tool electrode and pulse generator circuit was proposed in this research. This method uses the electrostatic induction feeding method where a pulse voltage is coupled to the working gap through a capacitance. The switch between the EDM and ECM modes was realized by utilizing the oxide film formed on the surface of the tungsten electrode. To switch the ECM mode to the EDM mode, the polarity of the diode attached in parallel to the working gap was changed to control the generation of the oxide film. In the ECM mode, the diode was connected so that it is forwardly biased when the polarity of the tool electrode is positive, thereby the oxide film was not formed on the tungsten electrode. In the EDM mode, since the polarity of the diode was reversed, the oxide film was formed, thereby discharge could be ignited. The switching ability was verified from the waveforms of gap current and voltage which showed that discharge occurred in the EDM mode, whereas there was no discharge waveform observed in the ECM mode. In the ECM mode, the influences of total amplitude and rise/fall time of pulse voltage were investigated. In the EDM mode, the discharge probability with different concentrations of electrolyte was researched first. Then, the influence of the feeding capacitance C1 on the EDM mode was investigated. Finally, through-holes were machined by conducting the EDM and ECM modes in this order to obtain higher material removal rate in rough machining and better surface roughness without residual stress in finish machining.

This paper aims to visualize spatio-temporal phenomena in electro physical and chemical processes in order to understand machining mechanisms and to achieve a technological breakthrough. In situ measurement methods to visualize the time evolution of stress, strain, temperatures, fluid velocities, and species density distributions, together with 2D or 3D images of the workpieces being processed are summarized. Applications of these methods to fundamental studies on electrical discharge machining, electrochemical machining, laser processing and additive manufacturing are introduced. Signal recording and processing technologies and real time monitoring enabling closed-loop feedback control are also discussed.

In the interelectrode gap of ECM, the electrolyte flow results in ununiformly distributed temperature and volume fraction of bubbles, leading to uneven distribution of the gap width. This paper aimed to realize high precision ECM using stationary electrolyte. A single current pulse was supplied after every jump flushing motion of the tool electrode. The pulse duration used was set sufficiently short not to cause the boiling of the electrolyte. Furthermore, the stamp flushing method was newly developed to squeeze out the sludge and bubbles from the gap to achieve high precision machining.

20th CIRP Conference on Electro Physical and Chemical Machining, ISEM 2020, online, 19 Jan 2021 - 21 Jan 2021; Procedia CIRP 95, 189-194 (2021). doi:10.1016/j.procir.2020.01.198 special issue: "20th CIRP Conference on Electro Physical and Chemical Machining / Edited by Konrad Wegener, Moritz Wiessner", Published by Elsevier, Amsterdam [u.a.]

The present study proposes a novel method for detecting micrometric undercuts (UCs) generated by electrochemical machining for the production of surfaces with tailored functionality. Two different algorithms for the detection of UCs based on two-dimensional topographic maps are tested. The first is a traditional approach based on definition of UCs in terms of surface orientation with respect to a reference direction. The second is an innovative alternative approach designed to reduce sensitivity to numerical effects that potentially lead to overestimation of the number of detected UCs. Electrochemical Jet Machining (ECJM) is used to texture SUS 316L specimens with the aim of producing a measurable surface with a representative number of micrometric UCs. Generated surface textures, comprising craters with diameters ranging from a few microns to tens of microns, are cross-sectioned and inspected with Scanning Electron Microscopy. The extracted profiles allow the novel method for detection of UCs to be efficiently tested and compared with the traditional approach. The number of UCs is found to decrease with increasing electrolyte jet scanning speed, while remarkable differences are revealed between the two calculation approaches at scanning speeds below 2 mm s−1.

A new method, wire electrochemical grinding(WECG), is proposed for machining tungsten micro rods with neutral electrolyte and bipolar current. The inevitable problem of tool wear due to the bipolar current can be eliminated by using a running wire as the tool electrode with this method. In this study, three kinds of wire guides were used for feeding the workpiece rod in the axial direction: cylindrical cemented tungsten carbide (WC) guide, cylindrical zirconium oxide (ZrO2) guide, and disk-shaped cemented WC guide, to investigate the influence of materials and configurations of the guide on the machining characteristics. The experimental results showed that the disk-shaped WC wire guide can realize higher machining accuracy because of less influence of stray current flowing through the gap formed between the machined micro-rod and wire guide. Even if the ZrO2 material has a much lower electrical conductivity than that of the cemented WC, the machining process was still influenced by the stray current between the micro-rod and wire guide. Furthermore, micro-rods were also machined by feeding the rod workpiece in the radial direction. Compared to when the workpiece is fed in the axial direction, the current was higher, which was clarified by the simulation of the current density distribution. However, the machining time was much longer than that with the workpiece fed in the axial direction because more machining steps were necessary to obtain a smooth side surface. With the workpiece fed in the axial direction method, a tungsten micro-rod of 35 μm in diameter was machined to the length of 163 μm. The results confirmed that the new method is capable of miniaturization equivalent to micro electrical discharge machining (EDM).

Recently, a multi-wire electrical discharge slicing (EDS) process has been proposed for slicing SiC ingots into wafers. A significant reduction in kerf loss is expected with this method compared with the conventional multi-wire saw method. However, this process entails a high risk of wire breakage. Therefore, in the present study, a novel electrical discharge slicing process utilizing a running ultra-thin foil tool electrode is demonstrated for the slicing of SiC ingots. Relative to multi-wire EDS, the risk of tool breakage can be reduced with this new technique by increasing the tool cross-sectional area. A 25-mm (1-inch) SiC ingot was successfully sliced with the proposed method and a kerf loss of about 100 μm was achieved by utilizing a foil tool electrode with a thickness of 50 μm. This paper summarizes the specific characteristics and results of this process, including the machining stability, machining strategy, selection of foil tool material and multiple simultaneous slicing.

Machining of high-aspect ratio slots is a common operation in industry, particularly in the mold and die and aerospace sectors. Electrical Discharge Machining (EDM) is a competitive technology for this operation, since it does not depend on material hardness and can fulfill strict geometrical requirements. However, due to debris accumulation in the narrow gap, the machining depth for stable machining is limited. The present work includes original findings about the influence of machined holes on flushing efficiency. Different configurations for this basic approach have been proposed and their performance discussed in terms of machining time and electrode wear. A remarkable and original finding is that continuous pockets or open holes are much better than the separated or closed holes. Furthermore, the paper shows that a sudden change in material removal rate occurs when the vacant spaces of the holes are sunk in the slot. A deeper insight into the EDM phenomena has been achieved through a study of discharge parameters, by analyzing discharge frequency, duty factor, discharge delay time and discharge off-time. In this study, using the proposed electrode reduced the process time by as much as 65% in the machining of 10 mm depth slots. Moreover, machining stability can be guaranteed if the flushing pockets machined on the electrode go through the whole depth of the machine.

The gap phenomena of EDM in deionized water and oil were observed using a high speed video camera. It is known that depending on situations, higher material removal rate can be obtained in water than in oil. To understand the reason, the distribution of discharge locations was observed using a transparent electrode made of SiC single crystal. It was found that the discharge locations were dispersed more widely in water than in oil. Hence, the machining was performed more stably in water than in oil, whereas the material removal volume per single pulse discharge was the same both in water and oil. Thus, the movement of bubbles and the flow field of the dielectric liquid in the working gap were observed using the same setup. Then, it was found that the radial flow of the liquid due to discharge can propagate with longer distances from the discharge spot and the oscillation of bubbles can continue for a longer period of time in water. Using oil in contrast, the radial flow could not reach a wider range than in water, and the bubble oscillation was damped due to the viscosity of oil, which is higher than water. Therefore, the temperature at the discharge spot can be lowered by the flushing flow of the liquid generated by the subsequent discharges. Thus, the cooling effect of the liquid is higher in water than in oil, thereby higher discharge current with shorter discharge interval can be used in water without causing a concentration of discharge locations. This is presumably the reason for the higher material removal rate in water than in oil.

In this study, tungsten was machined successfully with the neutral electrolyte NaNO 3 aqueous solution and bipolar current because the nonconductive oxide layer on the surface, which interrupts the electrochemical dissolution process, was removed by the NaOH generated when the polarity of the tungsten is negative. However, the tool electrode was worn due to the bipolar current. Although the tool wear ratio was decreased when the platinum sheet was used as tool electrode than that with the stainless steel plate used as tool electrode, the platinum tool was also slightly worn. Hence, to avoid the influence of the tool wear, a new method, WECG (wire electrochemical grinding), for tungsten machining with neutral electrolyte and bipolar current is proposed, and the running brass wire is used as the tool electrode. Thereby, the problem of tool wear inevitable due to the bipolar current was eliminated because the running wire tool was used in this method.

This paper reveals the influence of easiness of boiling of tool electrode materials on the machining speed in EDM. It is considered that during the discharge process, both the anode and cathode emit the metal vapor jets into the gap. The interaction of the jets in the gap generates a kind of shear force acting on the molten pool, which helps remove the molten material in the workpiece. Moreover, it was found that lower boiling point of tool electrode results in stronger interaction of the jets and larger shear force, thereby the material removal volume of the discharge crater will be larger. To validate the above thoughts, the effect of the interaction of the jets on the material removal process was investigated through the molecular dynamics simulation. Then, the material removal volume of the discharge crater in a series of single discharge experiments using tool electrodes with different thermo-physical properties related to the easiness of boiling was compared to verify the simulation results. Finally, the machining experiments were performed using a sinking EDM machine to verify the higher removal performance of tool electrode materials which are easy to emit vapor jets.

Electrolyte jet machining (EJM) is an innovative form of electrochemical machining (ECM) with excellent machining flexibility and accuracy. Recently, the nozzle inclination is introduced into EJM to further expand its machinability. In this work, to clearly reveal the mechanism behind this EJM process, a three-dimensional (3-D) multiphysics model of the stationary EJM process accounting for two-phase flow field and electric field was developed for the first time and experiments were also performed for validation. With the inclined nozzle, the distribution of flow velocity and thickness of electrolyte film around the jet exhibits uneven, thus resulting in non-circular hydraulic jump and non-axisymmetric anodic current density distribution, respectively. Besides, the effects of nozzle inclination angle and electrolyte ejecting flow velocity were investigated. Larger inclination angle leads to increased asymmetry of flow velocity and current density distribution, which results in more seriously non-circular hydraulic jump and asymmetric machined dimple shape. The electrolyte ejecting flow velocity affects the jet shape and current density distribution that eventually influences the machined results with the inclined nozzle, but this effect does not exist with the conventional vertical nozzle. Moreover, this work was extended to the translating EJM process to study the effect of nozzle translating direction with the inclined nozzle. It is demonstrated that through selecting translating directions, grooves with different shapes and surface finish can be machined even using a certain nozzle inclination. The mechanism for these phenomena was clarified by simulation, paving the way for design of the EJM process with the inclined nozzle.

Micro-rods were machined by electrochemical machining using the electrostatic induction feeding method, with which ultra-short current pulse duration of several tens of ns can easily be obtained. A tungsten plate and stainless steel (SUS304) rod were used as the tool electrode and workpiece, respectively. To improve the machining accuracy, the machining characteristics when the workpiece is fed in the axial and radial directions were investigated using NaCl aqueous solution as the electrolyte. When fed in the axial direction, the machinable length of the micro-rods was found to peak at the optimum feed speed because of the influence of pitting corrosion and collision between electrodes. When the workpiece was fed in the radial direction, the influence of pitting corrosion decreased, however, the micro-rod was shortened with increasing feed distance in the radial direction because of the stray current flowing through the end of the micro-rod. The simulation results of the material removal process agreed qualitatively with experimental results. Next, machining characteristics were compared between the electrolytes, NaCl and NaNO 3 aqueous solutions, by feeding the workpiece in the axial direction. It was found that the influence of pitting corrosion was eliminated with the NaNO 3 aqueous solution, and there was no machinable length limitation with suitable feed speeds. In addition, the taper angle and gap width were smaller with the NaNO 3 aqueous solution, compared with those of the NaCl aqueous solution. Stainless steel micro-rods of 100 μm in diameter with a high aspect ratio of 20 were fabricated with the NaNO 3 aqueous solution. According to the preliminary research results, the machinable minimum diameter of the micro-rods was investigated and micro-rods with an average diameter of 9 μm and length of 78 μm were machined successfully.

Electrical discharge machining by foil electrode serves as an alternative method for SiC slicing. This technology uses a highly tensioned thin foil as the tool electrode. The main advantages over wire EDM are that the foil thickness can be made smaller than the wire diameter, vibrations can be avoided by applying high tension, and higher current can be supplied since there is less risk of tool breakage. However, due to the large side surface area of the foil electrode, there is a high occurrence probability of side surface discharges and high concentration of debris, which affects kerf width accuracy and machining stability. In the aim to overcome both problems, this study proposes two foil electrode designs: a foil electrode in which holes are machined and the insulation of the side surface areas by a resin coating layer of 5 μm thickness. The influences of both foil electrodes were tested with three different slicing strategies: no strategy, applying jump motion of the tool electrode, and applying reciprocating motion. From machining experiments and comparative studies of the discharge delay time, it was found that with both foil tools, the occurrence probability of side surface discharges can be reduced. In addition, the chip pocketing effect of the holes enhance the flushing conditions, resulting in a higher cutting speed.

This paper describes tungsten micro-rod machining using a neutral electrolyte and ultra-short-pulse bipolar current supplied by the electrostatic induction feeding method. The tungsten oxide layer can be removed by NaOH generated when the polarity of tungsten electrode is negative. The machining accuracy improves with higher electrolyte concentration, and there is less tool wear when platinum tool electrode is used. A micro-rod with a diameter of 7.1 μm and aspect ratio of 14 was fabricated successfully. The machinable minimum diameter was equivalent to that obtained by EDM. Furthermore, the current waveform could be optimized to increase the material removal rate.

Micro-rods were machined by electrochemical machining using the electrostatic induction feeding method, with which ultra-short current pulse duration of several tens of ns was obtained. A tungsten plate and stainless steel (SUS304) rod were used as the tool electrode and workpiece, respectively. The rotating workpiece rod was fed axially in the direction normal to the top surface of the tool electrode, thereby the diameter of micro-rod was determined by the depth of cut in the radial direction. The influences of voltage amplitude and the thickness and surface area of the tool electrode on machining characteristics were investigated from the aspects of machinable length limitation, straightness of the micro-rod, pitting corrosion, and surface finish to determine the optimum machining conditions. The results of machining experiments and current density simulation showed that the influence of stray current on machining accuracy markedly decreased with decreasing tool electrode thickness, allowing micro-rods with higher aspect ratio and better surface finish to be fabricated. On the other hand, the influence of the top surface area of the tool electrode on machining accuracy was small due to small changes in the current density on the frontal surface of workpiece, resulting in small changes in the material removal rate (MRR).

This paper researches about the machining characteristics and servo feeding control of micro electrochemical machining (ECM) with the electrostatic induction feeding method. Since current can flow only in the rising and falling time of pulse voltage, short pulses in the order of several tens of nanoseconds can easily be obtained, realizing significantly narrow gap widths. To avoid short circuit in the small working gap of several micrometers, a servo feeding control system based on monitoring the peak of gap voltage was developed. Compared with the old method, which monitors the average gap voltage, controllability of the working gap width with higher response was obtained because of the higher S / N ratio. The influence of the reference voltage on the material removal rate (MRR) and machining accuracy was investigated. The results showed that higher MRR and smoother surface can be obtained with lower reference voltages, due to the higher current density in the working gap obtained with higher current efficiency. However, the MRR decreased when the reference voltage was too low, as a result of the interruption of machining by the frequent retreat of the tool electrode due to short circuit. On the other hand, the MRR increased with increasing feeding capacitance C 1 , because electric charge per each pulse duration increased in proportion to C 1 . The inlet side gap width was independent of C 1 , due to the higher MRR and shorter machining time with larger C 1 . Based on the preliminary study, through-holes of 50 μm in diameter were machined on a stainless steel (SUS304) plate with a thickness of 50 μm to investigate the straightness of the holes. The diameter of the hole at the inlet and outlet sides was 58.5 μm and 55.5 μm, respectively.

Slicing of single crystal silicon carbide (SiC) wafers by commonly used abrasive wire saw method is very time and cost consuming due to its extreme high hardness. This problem becomes more serious with the increase of SiC ingot diameter. Therefore, electrical discharge machining (EDM), which is a thermal process regardless of workpiece hardness, is developing as an alternative method for slicing SiC. In this study, EDM of SiC by a foil tool electrode was proposed for slicing SiC. Comparative experiments of foil EDM of SiC between in deionized water and in EDM oil were conducted. The tool electrode wear ratio, area cutting speed, kerf width, and surface integrity were experimentally investigated, and the reasons for the differences were discussed. It was clarified that foil EDM in deionized water can attain a higher cutting speed than that in EDM oil under the same machining conditions. However, the surface roughness and tool wear ratio were larger than in EDM oil. Short pulse duration was found to be more suitable for EDM of single crystal SiC in both dielectric liquids.

This paper describes a machining method of micro-rod using electrochemical machining (ECM) with the electrostatic induction feeding method developed by the authors. The influences of the feeding directions of workpiece rod on the machining characteristics were investigated. Higher voltage amplitude was needed to keep the same feed speed with the radial feeding method due to the larger machining area, and longer micro-rod was easier to be machined with the axial feeding method, because the jetting of electrolyte into the surface gap was easy. The influence of pitting corrosion on the machining accuracy was insignificant with the radial feeding method, because the side of the micro-rod is machined at the same time.

This paper describes the implementation of controlled pulse train method (CPTM) and CPTM considering resonance in the circuit. This work was performed using the setup of non-contact electrical feeding micro EDM with high speed tool electrode rotation. In this setup, a cylindrical feeding electrode is fixed coaxially to the tool electrode holder. Capacitance C 1 is formed between the two surfaces, realizing the non-contact electrical feeding and permits the tool to be rotated at high speed without vibration. Using this setup however, since C 1 is very small, machining with large discharge energy is difficult. Hence, CPTM was developed to allow several discharges to occur within the pulse train duration. Thus, larger diameter of crater was achieved. Furthermore, CPTM considering resonance in the circuit resulted in higher average discharge energy per pulse. As a result, largest diameter of discharge was achieved under the same C 1 . An observation of discharge location through a transparent electrode from the direction normal to the machining surface was conducted using a high-speed video camera. The results show that discharges within a pulse train duration are concentrated at the same location even if there is an interval between the discharges.

In electrochemical machining (ECM) processes, bubbles and sludge of metal oxides are generated in the gap. These products may influence the conductivity of electrolyte and the current distribution on the surface of workpiece, which directly influence the machining accuracy of ECM. In order to improve the machining accuracy, it is necessary to elucidate the gap phenomena of ECM. In this study, to investigate the mechanism of the change in the current density distribution, the gap phenomena were directly observed from the direction normal to the machining surface by a high speed video camera through the electrode made of transparent material such as SiC and ITO glass, which are optically transparent and electrically conductive. The influence of characteristics of transparent electrodes on the observation phenomena were compared. Moreover, the similarity of observed phenomena using different transparent electrodes to the actual ECM process was discussed.

Electrolyte jet turning system was developed to machine cemented tungsten carbide rods. Use of flat electrolyte jet enabled easy positioning of the jet relative to the workpiece rod and higher material removal rate than cylindrical jets. Using a bipolar current pulse, both cobalt binder and tungsten carbide were dissolved evenly. Influence of pulse conditions on the current efficiency and machining accuracy was investigated to obtain the optimum pulse conditions. Finer grain size of tungsten carbide resulted in higher material removal and smoother surface. A micro rod of 36 μm in diameter and aspect ratio of 20 was successfully machined.

To understand the influences of bubbles generated in the machining gap on stability and accuracy of electrochemical machining, gap phenomena were observed directly using a transparent electrode made of SiC single crystal, which is electrically conductive and optically transparent. The criterion of boiling for electrolyte was obtained from both experiment and heat transfer analysis, and the relationship between the occurrence of electric discharge and the volume fraction of bubbles in the gap was clarified. The influences of the rotation of the tool electrode and the radial flow of electrolyte in the gap on the machining accuracy were also investigated.

Single crystal silicon carbide is a promising material for producing next-generation power electronic devices owing to its outstanding physical properties of wide band gap, high dielectric strength, high heat resistance and high electron saturation drift velocity etc. However, slicing of SiC wafer is very time and cost consuming with conventional abrasive multi-wire saw method due to the extremely high hardness of SiC, especially when the diameter of SiC ingot becomes larger. In recent years wire EDM slicing method is being developed as an alternative method for wafering SiC ingots. However, wire vibration and wire breakage problems are inevitable in the process. Wire vibration causes a low cutting accuracy and wire breakage results in a low cutting efficiency. In this study, aiming to solve these problems and improve the EDM slicing performances of SiC, a new EDM wafering method which utilizes a blade electrode is proposed. The slicing process was realized by applying a relative motion between a tensioned blade tool electrode and a workpiece making use of a reciprocating worktable. The development of the experimental setup and the machining experiments are described in detail in this paper. The tool wear ratio, slicing speed, and kerf loss were investigated through slicing experiments and the characteristics of this new method are discussed.

Titanium alloy is increasingly used in industries due to its outstanding properties but machining of such difficult-to-cut metal faces lots of challenges. As an advanced electrochemical machining (ECM) method, electrolyte jet machining (EJM) is a potential alternative for machining of titanium alloy. However, oxide layer usually forms in traditional water-based electrolyte, which prevents uniform dissolution and consequently deteriorates the machining accuracy. To overcome the oxide layer, a novel electrolyte with organic solvent, NaCl ethylene glycol (EG)-based solution, was introduced in EJM of a typical titanium alloy (Ti1023) in this work. The current efficiency curves in NaCl water-based and EG-based electrolytes were experimentally measured, which show opposite behavior. Besides, machining experiments were conducted using different electrolytes. The results show that NaCl EG-based electrolyte helps to avoid formation of oxide layer throughout the EJM process, leading to better comprehensive performance than water-based electrolyte, especially in superimposed translating EJM. In addition, parametric effects on machining performance using NaCl EG-based electrolyte were investigated as the hint for selecting optimum machining conditions. Furthermore, high electrolyte temperature was proposed in this work to enable usage of small nozzle in EJM with EG-based electrolyte for miniaturization of machined geometry.

A new technique of EDM coring of single crystal silicon carbide (SiC) ingot was proposed in this paper. Currently single crystal SiC devices are still of high cost due to the high cost of bulk crystal SiC material and the difficulty in the fabrication process of SiC. In the manufacturing process of SiC ingot/wafer, localized cracks or defects occasionally occur due to thermal or mechanical causes resulted from fabrication processes which may waste the whole piece of material. To save the part of ingot without defects and maximize the material utilization, the authors proposed EDM coring method to cut out a no defect ingot from a larger diameter ingot which has localized defects. A special experimental setup was developed for EDM coring of SiC ingot in this study and its feasibility and machining performance were investigated. Meanwhile, in order to improve the machining rate, a novel multi-discharge EDM coring method by electrostatic induction feeding was established, which can realize multiple discharges in single pulse duration. Experimental results make it clear that EDM coring of SiC ingot can be carried out stably using the developed experimental setup. Taking advantage of the newly developed multi-discharge EDM method, both the machining speed and surface integrity can be improved.

This paper describes the observation of EDM gap phenomena through a transparent electrode from the direction normal to the machining surface using a high-speed video camera. To visualize the gap, SiC single crystal was used as the electrode material since it is optically transparent and electrically conductive. It was found that more than 70% of the working surface was occupied by bubbles generated by discharge after a few hundred pulse discharges from the start of machining. Electrical discharge occurs not only in liquid but also in bubble and at the boundary between liquid and bubble. The probability of discharge occurrence per unit area of each atmosphere was highest at the boundary between liquid and bubble. Furthermore, the probability of discharge occurrence through debris particles located at this boundary generated by the last discharge was highest.

This paper describes a mirror-like finishing technique by electrolyte jet machining. When the jet is still, the workpiece area where it collides into can be selectively finished to a mirror-like surface due to high current density at the center of the jet. When the jet is being translated, the low current density in the radial flow of the impinging jet deteriorates the surface roughness while the jet is passing over the surface. This problem can be resolved by reciprocating the jet at a high translating speed. Pulsed current and bipolar pulse also realizes mirror-like finishing even at low translating speeds.

Electrical discharge machining (EDM) is developing as a new alternative method for slicing single crystal silicon carbide (SiC) ingots into thin wafers. Aiming to improve the performance of EDM slicing of SiC wafers, the fundamental characteristics of EDM of SiC single crystal were experimentally investigated in this paper and compared to those of steel. Furthermore, EDM cutting of SiC ingot by utilizing copper foil electrodes was proposed and its performance was investigated. It is found that the EDM characteristics of SiC are very different from those of steel. The EDM machining rate of SiC is higher and the tool wear ratio is lower compared to those of steel, despite SiC having a higher thermal conductivity and melting point. Thermal cracks caused by the thermal shock of electrical discharges and the Joule heating effect due to the higher electrical resistivity are considered to be the main reasons for the higher material removal rate of SiC. It is concluded that the new EDM cutting method utilizing a foil electrode instead of a wire electrode has potential for slicing SiC wafers in the future.

This paper describes the optimization of the tool design and structure in an electrochemical curved hole machining method. Curved holes can be machined using a flexible tool which can be curved by the hole being processed by the tool itself. The curvature of the hole can be determined by the tilt angle of the electrode tip attached to the end of the tool. The tool design was optimized by a numerical analysis of the electrostatic field and a large deflection to reduce the radius of curvature and to improve machining efficiency. Machining tests showed that the curved hole shape is controlled by the detailed electrode shape and the feed rate of the electrode. The calculated curved hole shape agreed with the form of the holes machined experimentally.

EDM is a useful process for machining high-aspect ratio features with good accuracy in electrically conductive materials irrespective of their mechanical properties. With the ability of micro-EDM to compete with the resolution of conventional semi-conductor processing techniques, the process has attracted interest for the potential machining of single-crystal silicon. In order for the process to be feasible, the damage mechanism occurring during machining must be characterised to assess the need for secondary processing. Despite this the microstructural transformations induced by the process on the surface of the workpiece have not yet been assessed. In this study transmission electron microscopy (TEM) and laser-Raman spectroscopy are employed to characterise the microstructural changes as well as the presence of any contaminants and defects at the nano-scale. A twinned-crystalline structure created by epitaxial growth is formed in the recast layer. Some amorphous phase is also present. Findings indicate sub-surface pores between 10nm and 200nm diameter formed by gas expansion are observed. If the formation of such pores can be generalised for EDM processing of other materials, this phenomenon may contribute to the reduced mechanical integrity of such machined surfaces. Significant tool electrode material deposition with crystals of down to 3nm diameter also occurred in the workpiece surface. The nano-scale of embedded material may have implications for the progress of electrical discharge machining as a coating process and the properties of such coatings.

The modelling of ECM and EDM processes requires not one but several models to simulate the different phenomena that occur during machining. This paper reviews the models that have been developed to simulate each of these phenomena, e.g. potential models to calculate the current density distribution in ECM, thermal models for the plasma arc in EDM, moving boundary models to simulate the anodic dissolution in ECM and probabilistic models to determine the discharge location in EDM. In addition to discussing the relative merits of the techniques deployed in these models, the paper describes some salient applications and concludes with desirable future enhancements to these models.

This paper describes micro electrochemical machining (ECM) using the electrostatic induction feeding method. In ECM, electrolytic dissolution can be localized in the area where the gap width is narrow by using pulse durations shorter than several tens of nano-seconds. With the electrostatic induction feeding method which has been developed for micro electrical discharge machining, the current pulse of such short durations can be obtained more easily compared with the conventional pulse generators. In this study, the influences of the pulse voltage of power supply and feeding capacitance on the machining current were investigated theoretically. It was found that the current pulse duration is nearly equal to the rise time and fall time regardless of the pulse-on time of the pulse voltage. Hence, ultra-short pulses can be obtained without a need to use an expensive pulse generator. Micro-hole drilling carried out in a sodium nitrate aqueous solution with current pulse duration of 30ns showed that significantly small side gap of 2μm could be obtained.

In wire electrical discharge machining (WEDM), discharge current is one of the important factors which determine the machining properties, such as machining speed or machining accuracy. Previously developed machining simulators of WEDM, however, neglected the influence of workpiece conditions on the discharge current. Hence, this study developed a simulation tool for analyzing the discharge current in WEDM, where the electromagnetic field analysis around the wire electrode was coupled with the analysis of the equivalent circuit of the entire pulse generator circuit. The simulation results showed that, under the same pulse voltage and duration, the discharge current may change depending on the electromotive force which is determined by material properties, geometry and size of workpiece and wire electrode. In this study, the discharge current was calculated with different workpiece thicknesses. In addition, the discharge current was measured and compared with the calculation results in order to show the validity of the simulation tool.

This paper describes the development of a reverse simulation method for sinking electrical discharge machining (EDM) which can obtain tool electrode shapes with which desired workpiece shapes can be machined precisely taking into consideration tool electrode wear. In the reverse simulation method developed, the tool electrode is machined using a workpiece with the same initial shape as the target workpiece shape, applying the discharge location search algorithm developed for forward simulation. The data on the removal volumes of the tool electrode and workpiece per pulse, obtained from experiments, was switched in reverse simulation. To compensate for error due to curvature, the removal volumes of the tool electrode and workpiece were divided and multiplied by the ratio of the workpiece area facing a unit area of the tool electrode, respectively. Experimental results showed that the workpiece shape after actual machining using the tool electrode obtained from reverse simulation was closer to the target workpiece shape than when the tool electrode shape was obtained by offsetting the target workpiece shape at a distance equal to the gap width.

Micro and macro EDM are similar, but differences in machining conditions result in a differing power density, P e . This causes differences in the ratio of energy consumed for material removal with regard to energy distributed into the workpiece, and the ratio of total removal volume per pulse with respect to the molten area volume, so-called energy efficiency, E e , and removal efficiency, R e , respectively. It was found that, the P e of micro EDM is approximately 30 times greater than that of macro EDM. Consequently, E e and R e in micro EDM were significantly greater than those of macro EDM.

This paper describes the machining characteristics of high spindle speed micro EDM using the electrostatic induction feeding method. In this technique, non-contact electric feeding allows the tool electrode to be rotated at a high speed of up to 50000 rpm. In a previous study, it was shown that machining characteristics including material removal rate and accuracy can be improved by increasing the rotational speed. This is because rotation is effective in assisting flushing of debris particles from the gap and also cooling the tool electrode surface. This report investigates the influence of high spindle speeds on machining performance. As a result, it was found that much deeper micro-holes can be machined with better straightness and lower tool wear ratio for above reason.

This paper describes the development of an electrochemical machining method for cutting of metal sheets using an electrolyte jet. In this process the workpiece is machined locally in the area contacted by an electrolyte jet through which an electric current is applied. When holes and grooves are created by electrolyte jet machining, the electrolyte flow on the workpiece is influenced by changes in the configuration of the workpiece surface being machined, thereby broadening the current density distribution under the jet. Thus, in previous studies, depth of the slots was shallow, and perforation of holes or slots has never been investigated intensively. Hence, this study aims to perforate thick metal plates using electrolyte jet machining. Influences of the nozzle scanning speed and electrolysis current on the machining speed and hole shape quality were investigated. A stainless sheet of 3 mm in thickness was cut successfully with a kerf width of between 220 to 230 μm using a cylindrical electrolyte jet which was 210 μm in diameter.

This paper describes the development of a novel machining method capable of micro-milling and electrochemical turning using a flat electrolyte jet. The workpiece is machined locally in the area hit by the jet which moves when an electrical current is applied to it. Use of a flat jet in place of a cylindrical jet improves milling speed, and turning process is realized by the flat jet hitting the surface of the rotating cylindrical workpiece. Since depth of cut can be determined by the electrical current or dwelling time of the jet on the surface, there is no need for precise positioning of the nozzle against the workpiece.

Electrical discharge phenomena in EDM occur in a very short time period and in a very narrow space, thus making both observation and theoretical analysis extremely difficult. For this reason, the material removal mechanism in EDM has yet to be understood clearly. In this paper, the forming process of discharge craters in three dimensions was simulated, and material removal mechanism in EDM was analyzed using Molecular Dynamics (MD). It was found that material removal mechanism in EDM can be explained in two ways; one by vaporization and the other by the bubble explosion of superheated metals. It was also found that the metal removal efficiency is 0.02–0.05, leaving most of the melted pool resolidified. In addition, the influence of power density on the removal process was investigated, and the results showed that as the power density increases, the diameter and depth of the melted area increase, as does the metal removal efficiency. In this study, the forming mechanism of the bulge around discharge craters was also analyzed, and it was found that bulge is formed due to two mechanisms. The first is the shearing flow of the molten material caused by the extremely high pressure in the superheated material, and the second is the accumulation of the ejected material on the bulge formed by the first forming mechanism. It was also found that existence of micro pores in the workpiece material increases the depth of the discharge crater and melted area, thereby increasing the machining surface roughness. Simulation of the distribution of removed materials in the gap showed that some part of the removed material becomes debris ejected from the gap, while another part settles on the surface of the opposite electrode, and the last part returns to the surface of the electrode from which it was ejected.